The invention relates generally to a passive, liquid-crystal flat screen information center.
More specifically, a liquid-crystal matrix flat screen information center has a flat screen to which is attached an electronic control unit. Attached to the surface of the flat screen are a number of individual protuberances, called "Pixel." These Pixel are connected by means of conductors to the control unit and are capable of being switched between a translucent and a light-absorbing state. The flat screen is also fitted with two polarisation filters, one of which is located in the light path in front of, the other in the light path behind, the flat screen. The existing flat screens of this kind can be placed on the surface of an overhead projector on which material is placed to be projected onto a screen. Thus, the information on the flat screen can be transmitted in the form of writing, formulas, drawings, or the like, onto a screen. In the existing flat screens the polarisation filters are firmly attached to the actual flat screen. This has several disadvantages:
1. Present flat screens with a relatively large surface area require relatively large polarisation filters. The polarisation filters made of plastic sheeting which are usually employed have a limited life span, typically 600 hours, as the polarisation sheets are broken down by heat and emissions from the light source, especially by the ultra-violet component of the light source. As regards the existing flat screens, when the polarisation filters are no longer usable, they must be replaced and the flat screens must be replaced with them.
2. The flat screen itself is affected by heat. Sufficient projection contrast can be achieved only below 50.degree. C., and in the more modern indicators only below 40.degree. C. By reason of the light source which is required for projection, however, the polarisation sheets and the flat screen itself are also heated, the light source especially emitting a high IR-energy component. Most of the heat is absorbed in the lower polarisation sheet, that is, the one which faces the source of light, and the heat which is thereby generated is transmitted to the flat screen to which it is attached. Although the flat screen itself absorbs only a small amount of heat, this also contributes to an increase in temperature. Finally, a further amount of the light energy is trapped in the second polarisation filter and heats it. As the second polarisation filter is also attached to the flat screen, further heat is thus conducted to the flat screen. Until now, it has proven difficult to project liquid-crystal indicators onto large surfaces. In consequence of the transference of heat from the light source various pixel of the flat screen can, depending on the quality of the technology, change color even though they are not directed by the control unit.
Liquid-crystal indicators nevertheless offer a number of advantages which recommend them over all other indicators. These advantages include: low operating voltage, absence of shimmering, low weight, flat construction, transparency and resistance to interference by rays. Thus, their application in projection devices is worth serious consideration.
Proceeding from these premises, it is the purpose of the invention to improve the liquid-crystal indicators described above, but especially the liquid-crystal matrix-indicators, in such a way that they are suitable for projection. In this way, they are especially resistant to the heat to which they are subjected when they are placed on the projection surface of an overhead projector. This objective is achieved by a liquid-crystal indicator having the following characteristics: Both the polarisation filters are positioned on such a distance from the flat screen that the flat screen and each of the polarisation filters there is formed an air tunnel or canal in which there flows a current of air which impinges on the inner surfaces of the polarisation filters and the flat screen and cools them.
By reason of the distance between both the polarisation filters and the flat screen, the polarisation filters cannot directly transfer the heat which they absorb to the flat screen. Moreover, the polarisation filter can be exchanged independently of the flat screen. But of greatest significance is the fact that a space is thereby created between the flat screen and each of the polarisation filters, and through this space a current of cooling air can be made to flow. As the heat in the conventional flat screens, which are normally operated horizontally, cannot escape by convection currents, constant cooling, generated ideally by a fan, is achieved by the air flow thus produced. This cooling is of benefit not only to the flat screen, but also lengthens the life of the polarisation sheets.
In a preferred embodiment of the invention the flat screen is housed in a flat cover. The flat screen itself has approximately the same dimensions as the projecting surface of overhead projectors, that is to say, DIN A4 or A5.The housing is just a few centimeters high, for example, 40 mm. In consequence, the distance between the flat screen and the polarisation sheets placed above and below it is just a few millimeters, for example, 15 mm to 20 mm. In order that the cooling currents flow uniformly, it is advantageous to have the pressure side of the fan, which is situated in the housing, connected directly via exhaust ports to the outside and also have a number of intake ports in the housing through which the air can be drawn. In this way, the current of air can be controlled by means of the special arrangement of the intake ports and strategically placed air-flow guides. It is therefore possible to ensure that the middle of the polarisation filters and the flat screen are exposed to greater cooling, especially since more heat is being concentrated at these points than at any others. It is also possible to direct additional fresh air to those portions of the filter and the flat screen which are at the rear of the fresh air passage. In other words, it is not necessary to attempt to cool them with air which has already been warmed through contact with other parts of the sheets. It is proposed in further embodiments of the invention to attach at least one of the polarisation filters to a pane of glass or plastic, and attach this pane to the housing in such a way that it can quickly be detached and replaced. For this purpose, a frame would appear to be the idea solution, so that the pane attached to the polarisation sheet could be inserted into the frame. The frame can then be secured in place by locking springs, quick-release screws, removable clamps, or the like. It is important that the pane should be capable of being placed on the housing from outside, i.e., it should not be necessary to open the housing in order to remove the pane and the polarisation sheet.
It is of considerable advantage to place the polarisation filter on the housing in such a manner that it can be rotated. In this connection, it is sufficient to have a rotations angle of normal dimensions, e.g., 150.degree.. Thus, rotation permits a variation of the color in which the material to be projected is actually displayed.
It is also advantageous to attach a UV filter to the carrying plates for the polarisation sheets and the actual flat screen, in order to protect them from ultra-violet rays. The ultra-violet rays, which release considerable energy, are prevented from promoting chemical and other damaging processes. Furthermore, it is advantageous to attach an anti-reflection coating or sheeting to the surfaces of the polarisation filters, plates and the flat screen as in this way light reflection, and therefore light loss, is decreased.
Finally, it has also proved most advantageous to utilize the flat screen in such a way that, depending on the type of presentation, as many Pixel as possible are activated. For example, if it is desired to project text, not only those Pixel which form the actual letters should be activated, but the whole of the remaining surface can also be activated in negative projection. It has been shown in tests that activated Pixel are less susceptible during projection to being affected by heat and other influences. It is therefore recommended to use the flat screen in such a way that, depending on the type of projection desired, the greatest number of Pixel are activated so as to enhance the contrast. The desired projection onto the screen (negative or positive display) is achieved by use of the control unit. For examples, if the flat screen is being employed for inverse display, then the control unit is set or controlled in such a way that it operates inversely. The invention, it should finally be noted, intends to improve the flat screen itself. In a one-level flat screen, all the Pixel are located on the one level. The individual Pixel must nevertheless be isolated electrically from one another. In consequence, there must of necessity be open spaces between the individual Pixel. Nevertheless, in one-layer flat screens, it is not possible to totally black out or totally illuminate a surface. To correct this, it is proposed to provide the flat screen with at least two layers, and to arrange the Pixel in the second layer in such a way that they occupy the spaces between the Pixel of the first layer, while the Pixel of the first layer occupy the spaces in the second layer. Thus, it is possible to achieve with a two-layered flat screen the total blacking out or, if desired, illumination of a surface. There remain very small areas which do not have the same state as the Pixel. These remaining areas can, of course, be blacked out or illuminated by assigning Pixel to them from a third layer of the flat screen. The flat screen is therefore so arranged that the flat screen surface when viewed vertically reveals that the Pixel of the various levels constitute as closely and as totally as possible a closed or dense surface, e.g., one which is free of gaps. Where two or more layers (information levels) are joined together as as to exclude spaces, those gaps are eliminated which invariably appear between the Pixel of a diagram or projection of liquid-crystal indicators. If in place of the polarisation filters or, in addition to them, colored filters are used, these two are located at a distance from the indicator unit and so are placed in the cooling air current.